There is a thrust in industry to produce high quality products consistently. This is also the case in the paper industry. Particularly, the paper industry is concerned with the print properties of paper. The coated paper industry faces several major problems in determining these print properties. The printing of paper usually takes place remote from the mill. Generally large quantities of paper have been prepared before the paper is actually commercially printed. To try to minimize these problems a series of laboratory tests have been developed to measure the print properties of paper including print gloss. Unfortunately, the most accurate test is to condition a sample of paper (usually overnight), print it, and then test it. Paper mills run at speeds of about 1500 m/min. Even if the testing could be completed in 1 or 2 hours there is a potential to produce huge amounts of poor quality material. There is a need to develop a simple, low cost, portable test which will relatively quickly indicate the potential print gloss of coated paper. Due to the high speed of the paper web it is not anticipated that the type of profilometer used in the present invention could be used continuously on the paper coater.
A method to test the surface smoothness of paper does exist but it is not portable, and it is expensive. The currently accepted method in North America is the Parker Surface Roughness Test (Print Surf) method. This method measures the air which passes between the surface of a coated paper and a flat surface of either steel or rubber. The apparatus to carry out such a test costs about $20,000 (CDN.). The equipment is not portable and requires laboratory staff and space. More importantly the test lacks a high resolution for the smoothest surfaces. The present invention uses a hand held portable device which has high resolution and costs about $4,000 (CDN.).
Portability is important from a production point of view as it provides immediate information. For example, the present invention contemplates that the surface profile of paper in jumbo rolls in the finishing area could be measured in the storage area. The read out of the profilometer is immediate and may be related to a number of properties including predicted print gloss.
Some initial work in this area was done by Kimberly-Clark Corp. in the early 1950's. This work was presented in "Effects of Smoothness and Compressibility on the Printing Quality of Coated Paper" Walter W. Rocher TAPPI Vol. 38 No. 11 November 1955 pg. 660. The paper does not seem to contemplate or suggest relating the measurements taken to print gloss or other properties of the paper. Additionally, the device used does not appear to be portable or hand held.
There has been some work on characterizing paper using profilometry. One of the current leaders in this field is English China Clays International which uses a modified, non-portable, Talystep (trade mark) profilometer instrument. The data is analysed using Fourier transforms to segregate different types of periodic occurrences. These then may be attributed to macroroughness (e.g. the roughness of the base sheet) and microroughness (e.g. the roughness of the coating). The data may also be used to generate computer plots of the surface topography of coated paper. ("Applications of Novel Techniques for Quantitative Characterisation of Coating Structure" H. J. Kent et al TAPPI 1986 Coating Conference pg. 103)
A paper presented at the TAPPI 1984 Coating Conference (pg. 149) "Board Surface Structure and Gravure Printability" Per-Johan Aschan Tapio Makkonen and Jorma Paakko, the Finnish Pulp and Paper Institute, discusses the use of profilometry to classify the cumulative number of voids of a specified size which occur during a profile measurement (CN number) and their distribution (Sd). These data together with the Parker Surface Roughness for the base uncoated sheet have been correlated to the Heliostat print test values for printed board. In the paper it is proposed to predict the Helio test value as a function of both base board and coated surface smoothness. E.g. Helio=589.8-0.103 CN-99.2 BPPS-258.0 Sd+49.2 Sd.BPPS in which CN is the cumulative number of voids, BPPS is the base board smoothness (Parker Surface Roughness) and Sd is the distribution index for voids. Both CN and Sd are determined using a profilometer. The data suggest that profilometry is useful provided the base board surface smoothness is consistent (e.g. different curves for different base boards). The paper suggests that poor correlation is achieved using a representative range of papers and a profilometry device. The profilometry device used was built by the Finnish Pulp and Paper Research Institute and it appears that the device is not portable as the data is generated using lab samples two meters in length.
The present invention seeks to provide a rapid method to measure the surface profile of paper and relate this measurement to other properties of the paper and the printed paper using a portable (hand held) profilometer.